A great number of capacitive sensors have been heretofore developed for the sensing of persons or materials to provide an alarm, indicating signal, or control. For example, capacitive sensing circuits have been used for alarm systems to provide a signal in response to touching of a particular area or the proximity of an object. In other instances, capacitive sensing circuits have been utilized to detect the presence or absence of liquids and solids and thereafter initiating an indicator for alarm signals or measurement. Capacitive sensors have also been used to measure the distance to an object, material size, material moisture content, oil contamination, humidity, pressure, liquid level and in fact have formed the basis for sensing in numerous measurement and detection applications.
With regard to dispenser control, it is often preferable to operate a device without direct handling thereof by human interaction. For example, it is preferable for sanitary reasons in washing to avoid the need for physical contact with faucet handles, towel dispensers, hand dryers, soap dispensers and the like.
While a number of control systems have been developed for such touch-free devices in order to conserve water and soap, they have been plagued by false activation. That is, devices are turned on without the actual presence of a human body part. This, of course, leads to fluid waste that is contrary to the original purpose of the control system.
Further, in the case of soap dispensers and the like, safety becomes a factor when such liquids are falsely dispensed and accumulate on a floor, or other surface, where subsequent slippage thereon may cause bodily harm.
The problem of false activation, and more generally of reliable as well as sensitive detection of a proximate object by a proximity sensor, stems from the need to reliably discriminate between a small change in signal strength due to changes in the proximity of the object versus changes in signal strength which can occur due to other factors such as sensor noise, sensor drift or induced changes in the signal due to actual changes in the ambient environment itself, such as contamination of the sensor and other effects which can give rise to signals which are similar in magnitude to or even larger than the detection signal itself.
In the case of infrared proximity sensors, which are for instance frequently used in current commercial non-contact soap dispensers and other similar devices, false activation can arise due to the effects of stray, extraneous light impinging on the sensor due to spurious reflections in shiny objects or otherwise, or a failure to detect an object can occur due to variations in the reflectivity of the object or contamination of the optics.
In the case of capacitive proximity sensors, where an object is sensed via the detection of a change in capacitance due to the proximate presence of the object, sensitive detection of a proximate object in everyday environments is made difficult and unreliable because the actual capacitance changes due to a proximate object can be small compared with other changes in capacitance due to changes in the surroundings.
Certain commonly occurring variations in the environment which can cause such interfering variations in capacitance include contamination of the surface of the electrodes or other structures in the sensing field region by gradual dirt accumulation or condensed moisture, significant changes in ambient humidity, gradual variations in the proximity or composition of other nearby structures and objects, or variations in sensor mounting location, all of which can give rise to small alterations in the electric field shape or intensity between the sensor electrodes thereby altering the charge state and hence capacitance between the electrodes.
There are currently two basic types of capacitive proximity sensor in the known prior art. In one case, often referred to as the parallel plate type, there is only one sense electrode at the sensor and the capacitance to ground is measured. If the object to be sensed is generally conductive and grounded it can effectively form the second electrode such that movement of the object towards or away from the primary sense electrode changes the capacitance and this change is measured and related to the distance or proximity of the object.
If the object to be sensed is instead not electrically conducting, a second stationary electrode is incorporated at a fixed distance away and connected to ground and the object to be sensed is passed between the two electrodes giving rise to a change in capacitance. In the second case, called the fringe field type, there are instead two sense electrodes disposed near one another at the sensor and the object which is sensed changes the capacitance between them by changing the electric field by dielectric or conductive effects. The resulting change in capacitance is sensed and this can then be related to a change in distance or proximity of the object. Fringe field type capacitive proximity sensors are widely used industrially in manufacturing applications where sensor installations are typically specified and fixed, and other potentially interfering environmental factors can be controlled.
Such devices nevertheless also frequently incorporate an additional electrode to separately sense for and thereby compensate for drift due to surface contamination. The maximum sensing distance is the sensor range and this is related to the sensitivity of the capacitance change sensing technique, the nature and size of the object to be detected and the physical size of the sense electrodes. Larger sense electrodes provide greater range.
More sensitive detection provides greater range with a given electrode size and a given object to be sensed, which is a performance advantage in applications where larger electrode structures are undesirable and greater range is desired. However, more sensitive detection of changes in capacitance does not by itself provide reliability where significant capacitance changes can also arise due to environmental factors.